Unfinished promote may not break paused cluster.

When a node to promote dies before finishing the promote and the
cluster is in a standby mode, the failover key sticks indefinitely,
preventing any master to take over the leader role. Prevent it by
letting the node in a master role cleanup the failover key if the
node to failover is not present among the members.

The master check cannot be performed by the node role alone, since
the node will not change its cached role on a manual promote. We
need to check the DB state as well.

patroni/ha.py

@@ -237,6 +237,11 @@ class Ha(object):
             if failover.candidate == self.state_handler.name:  # manual failover to me
                 return True
             elif self.is_paused():
+                # Remove failover key if the node to failover has terminated to avoid waiting for it indefinitely
+                # In order to avoid race conditions only the master is allowed to do so.
+                if (not self.cluster.get_member(failover.candidate, fallback_to_leader=False) and
+                   (self.state_handler.is_leader() or self.state_handler.role == 'master')):
+                    self.dcs.manual_failover('', '', index=self.cluster.failover.index)
                 return False
 
             # find specific node and check that it is healthy